1. Tape Already Glows a Little. What Did the UCLA Team Add?
Scientists call light made by pulling, rubbing, or breaking things triboluminescence. The Nature paper begins by describing it as an optical phenomenon sometimes seen when two surfaces in contact move relative to one another [1]. Peeling sticky tape is one way to see it, and the researchers studied the light that comes off when tape is pulled from a surface [1].
What was new in 2008 was how far the light reached. According to the paper's abstract, peeling tape in a vacuum released energy all the way into the X-ray range, strong enough to take X-ray photographs [1]. X-rays are a kind of light, like the visible light of a lamp, but each X-ray particle of light carries much more energy than a visible one [2]. So the surprise was not that tape can glow. It was that it could produce something this energetic.
2. How Did the Team Test Tape in a Vacuum?
The team used a motor to unroll tape from a spool inside a small vacuum chamber [2][3]. Physics World reported a pressure of about one millionth of an atmosphere [3]. That is a strong vacuum by kitchen standards, but still far from the emptiness of outer space.
The detectors recorded X-ray bursts that each lasted about a billionth of a second [2][3]. A billionth of a second is to one second roughly what one second is to 32 years. One burst contained up to about 100,000 X-ray particles of light [2]. The bursts lined up with small drops in the force needed to peel the tape [3]. Peeling is not perfectly smooth: the tape grips, slips, and grips again, a stick-and-slip motion, and the X-rays flashed along with it [2][3].
3. Were the X-Rays Really Strong Enough to Photograph a Finger?
Yes, with limits. The team used the tape as the X-ray source and photographed a human finger. The exposure lasted 20 seconds [3]. In another account, the picture of the finger bones was recorded on dental X-ray film while the tape was peeled at about 3 cm per second (about 1.2 inches per second) [2][4][5].
The picture was not as sharp as a hospital X-ray. Team member Carlos Camara told MIT Technology Review the pictures were "taken with Scotch tape, so there's room for improvement" [4]. The group said it hoped the work might eventually lead to a cheap, simple X-ray source for medical use, and that a final device would not be expected to use adhesive tape [4].
4. Why Does Air Stop the Tape From Making X-Rays?
The explanation the team gave, as Physics World and Science News report it, has three steps [3][2].
- As the tape peels, the exposed sticky side picks up a positive electric charge, and the outer surface of the roll picks up a negative charge [3]. Tiny electrons, which carry negative charge, are left on the roll's surface [2].
- The charges pull on each other, and electrons jump across the gap toward the tape. A gap of about two-thousandths of an inch (0.05 mm) was mentioned in one report [5]. On the way, the electrons speed up [2].
- When fast electrons slam into the tape's surface and slow down suddenly, they give off energy as X-rays [2][3][5]. Science News reported an estimate of roughly one electron in 10,000 producing a high-energy X-ray particle [2].
Air gets in the way of step 2. At normal pressure, the particles in the air slow the electrons down, so they never reach the energy needed to make X-rays [2]. A simple way to picture it is a runner crossing an empty field compared with a crowded train station: in the field the runner can pick up speed, and in the station the runner keeps bumping into people. This picture only captures the idea that crowding slows things down. Real electrons behave in more complicated ways.
That is why the effect is tied to the vacuum chamber. In the reports, the X-rays were seen in the vacuum setup, and visible light is also produced when tape is peeled [4]. The air slowing the electrons is the explanation Science News gives for why [2].
5. What Is Still Unexplained About Tape X-Rays?
The explanation above leaves out something important. The Nature abstract says the energy and the very short flashes the team saw go beyond what current theories predict, which it calls a challenge for theorists [1]. Physics World, quoting the paper, said it is not understood how energy spread out over a large area of peeling tape ends up concentrated into nanosecond X-ray pulses, and described this as a "new inspiring mystery" [3].
Camara said that no current theory predicts that there will be as many charges as the observation requires, and that the team expected some X-rays but not this many [2]. The idea was not entirely new, though. A Russian experiment in 1953 had hinted at X-rays from peeling tape, but it was little known [2]. Another physicist, Enrique Cerda, guessed that the emission might not be limited to peeling tape and could also happen when a film is torn, but that was a guess and not a result [2]. Discover reported that the team speculated other kinds of tape might raise the X-ray output a great deal; that was not tested in this experiment [5].
6. How Can You Explore This Safely?
You cannot repeat the X-ray experiment at home, and you should not try: it needs a vacuum chamber and a motor-driven setup, and the reported X-rays came from the vacuum setup [4][3]. Ordinary tape on a desk is not an X-ray source.
What you can do is read the original abstract of the Nature paper [1], and ask better questions. For example: why might a very short flash be easier to produce than a steady glow? Would a different kind of tape give more or fewer flashes? And how would you test the idea that other materials might do the same? If you want a safe puzzle, compare how a piece of tape sticks to two clean surfaces at home, such as a plastic lid and a glass jar, and write down what you notice.